The present invention relates to method and apparatus for producing small radius drops from a larger radius orifice using a drop-on-demand drop dispenser, and more particularly from a dispenser that is piezoelectrically driven.
The use of drop-on-demand (DOD) inkjet technologies is becoming increasingly widespread in many industrial applications ranging from gene chip production to separations to paper printing. Since the development of the first DOD inkjet devices, great advances in inkjet technologies have made ink-jets economical and versatile. As popularity of ink-jets grows so does the need to understand the factors which contribute to drop quality (e.g. drop speed, accuracy, and uniformity). Additionally, gene chip arraying devices have the special requirement that they should be capable to dispensing many different types of liquids using a given nozzle, where a typical ink-jet printer may dispense only a single ink formulation per nozzle.
Currently, the most powerful methods by which to study DOD drop formation involve using direct visualization to examine effects of varying system parameters on drop quality. The simplest method to capture images of working DOD nozzles is by using stroboscopic camera systems. Stroboscopic techniques have been used for more than sixty years to image drop formation and interface rupture dynamics. More recently, the development of CCD cameras and diode lighting systems offers inexpensive methods of obtaining images at high temporal and spatial resolution. However, in order for stroboscopic analysis to be effective, the drop to drop variation in dispenses must be small. Stroboscopic methods cannot image the evolution in time of a single drop which can be a disadvantage when it is desirable to image only the first few drops.
Alternatives to stroboscopic cameras are high speed digital imaging systems which can acquire frames at rates from 1,000 to 100 million frames per second with exposure times ranging from milliseconds to nanoseconds. While slower digital imagers have been used extensively to study drop formation from capillaries at constant flow rate, they are too slow to capture the dynamics of inkjet drop formation which occurs on microsecond time scales. Ultrafast imagers (microsecond to nanosecond resolution) are relatively new and little work has been done with DOD ink-jet imaging using these cameras . Ultrafast imaging is a powerful tool which is useful not only for diagnostic imaging (e.g. evaluating drop quality), but can also be quite useful in designing DOD dispense methods as well as optimizing system performance. Digital camera systems now provide imaging at high speed and resolution with convenience and ease of use heretofore unparalleled. Changes in performance in ink-jet nozzles with varying system parameters can be observed very quickly in great detail so that time for system optimization can be minimized.
Redesigning existing ink-jet nozzles and developing novel dispensers is a route by which innovations in the drop dispensing technology can develop. However, a simple method to improve drop dispense quality is to adjust operating parameters (e.g. line pressure, electrical control signal, liquid properties) to produce optimal drop dispense.
Some DOD dispensing systems currently in use utilize electrical control signals with particular characteristics in order to achieve the desired drop qualities. For example, some existing systems use a control signal that consists of a waveform with a single polarity, such as half of a square wave. Yet other existing systems use an electrical control signal consisting of two portions, one portion being of a first polarity and the other portion being of a second and opposite polarity, such as a single, full square wave. In some cases, the timed durations of the two portions are identical. Many of these systems provide an electrical control signal that grossly produces one or more large drops, the large drops being created by a fluid meniscus which takes on a generally convex shape on the exterior of ejecting orifice. The large drop is formed when the edges of the meniscus in contact with the orifice separate from the orifice. These systems produce drops of a diameter equal to or greater than the diameter of the orifice. Yet other systems produce drops by resonating the meniscus. Such systems do not generally move the meniscus either toward the exterior of the dispenser, or toward the internal passage of the dispenser, but simply create oscillatory conditions on the meniscus. The drop quality of such oscillatory dispensing methods are likely to be subject to manufacturing imperfections near the orifice, or deposits of material near the orifice, such as dried ink.
The present invention overcomes these disadvantages in novel and unobvious ways.
One aspect of the present of the current invention concerns a method for expelling a drop of fluid from an orifice. The method comprises providing a body defining a passageway terminating at an orifice, with fluid being contained in the passageway proximate to the orifice, and the fluid forming a meniscus in the passageway. The method includes a first withdrawing of the fluid in the passageway in a first direction from the orifice. After said first withdrawing there is a propelling of the fluid in the passageway in a second direction opposite to the first direction and toward the orifice. After the propelling, there is a second withdrawing of the fluid in the passageway in the first direction. The withdrawing is continued for a time sufficient to retract a portion of the meniscus and after the retracting, a drop of fluid is expelled from the orifice.
Another aspect of the present invention concerns a method for expelling a drop of fluid from an orifice. The method includes providing a body defining a passageway terminating at an orifice, with fluid being contained in the passageway proximate to the orifice. There is a first withdrawing of the fluid in the passageway in a first direction from the orifice for a first duration of time. After the first withdrawing, there is a propelling of the fluid in the passageway in a second direction opposite to the first direction and toward the orifice for a second duration of time less than the first duration. After the propelling, there is a second withdrawing of the fluid in the passageway in the first direction. The method includes expelling a drop of fluid from the orifice.
Another aspect of the present invention concerns a method for expelling a drop of fluid from an orifice. The method includes providing a body defining a passageway terminating at an orifice, with fluid being contained in the passageway proximate to the orifice. The method also includes a first withdrawing of the fluid in the passageway in a first direction from the orifice. After the first withdrawing, there is a propelling of the fluid in the passageway in a second direction opposite of the first direction and toward the orifice for a first duration of time. After the propelling, there is a second withdrawing of the fluid in the passageway in the first direction for a second duration of time greater than the first duration. The method includes expelling a drop of the fluid from the orifice.
Another aspect of the present invention concerns an apparatus for ejecting a drop of fluid from an orifice. The apparatus comprises a body defining a passageway terminating at an orifice and a reservoir of fluid in the passageway, the fluid forming a meniscus in the passageway. The apparatus includes a piezoelectric actuator coupled to the body and actuatable to withdraw fluid in said passageway away from the orifice and actuatable to propel fluid in the passageway toward the orifice. There is also a controller providing a control signal to actuate the piezoelectric driver and including first, second, and third portions. The piezoelectric actuator withdraws fluid in the passageway toward the interior in response to the first and third portions, and propels fluid in the passageway toward the orifice in response to the second portion. The second portion follows the first portion and the third portion follows the second portion. The first portion retracts the meniscus from the orifice with a first velocity. The second portion propels the fluid in the center of the passageway toward the orifice with a second velocity greater than the first velocity. The third portion retracts the meniscus from the orifice, and an outward tongue of fluid forms on the meniscus after the retraction, separates from the meniscus, and is ejected as a drop.
These and other objects of the present invention will be apparent from the description of the preferred embodiment, the claims and the drawings to follow.